It is well known that polypropylene can be cross linked, partially cross linked and modified by the action of gamma radiation or by the use of electron beam accelerators with energies of 0.5 MeV to 10 MeV. Additions of 2 to 3% of electron-irradiated polypropylene to unmodified polypropylene act as nucleating agents for polypropylene, increasing the crystallization temperature and shortening the crystallization time. This leads to an increased degree of crystallinity, a higher density and a higher modulus of elasticity (Menges, D., Kunststoffe 73 (1983) 5, 258-260; Lee, D., Kunststoffe 81 (1991) 7, 609-613, Kirch, D. Plastverarbeiter 37 (1986) 10, 100-106).
It is furthermore known that modified polypropylenes with free long-chain branching can be produced by treating polypropylene first with high-energy ionizing radiation by electron beam accelerators with energies of 500 to 4,000 keV and radiation doses of 1 to 9 Mrad under predominantly inert conditions and subsequently treating the modified polypropylenes with heat (German patent 3,688,258, European patent 0,190,889, German patent 0,634,441, German patent 0,351,866, U.S. Pat. No. 4,916,198, U.S. Pat. No. 5,047,446). The use of mixtures of these modified, branched polypropylenes with unmodified polypropylenes for producing films, sheets, panels and coatings is likewise known (European patent 0,634,454).
The high safety expenditure for biologically shielding the high radiation is a disadvantage when modifying polypropylene with high energy, ionizing radiation with electron beam accelerators of more than 500 keV. This shielding requires expensive hermitization of the radiation chamber by thick walls of concrete or by lead segment constructions, expensive radiation protection measurement techniques, as well as medical monitoring of the operating personnel. In the case of mixtures of modified and unmodified polypropylenes, a further disadvantage of the present technology consists therein that an additional processing step is required to produce the mixtures.
The aim of the invention was to develop a continuous method for producing polypropylene mixtures of higher stress-crack resistance and melt strength, without increased expenditure for safety and without an additional processing step for producing the mixture.
Surprisingly, it was observed that, when very finely divided polypropylene powders and low energy electron beam accelerators are used, mixtures of unmodified and modified polypropylene particles result, which meet the requirements for polypropylene mixtures of increased stress-crack resistance and melt strength, despite the very slight depth of penetration of the electrons having low energy.